1. Field of the Invention
The present invention generally relates to devices for measuring the interior dimensions of hollow bodies and, in particular, for measuring bodies having a very restricted opening through which the measuring apparatus must pass.
2. Description of the Prior Art
Recently, there has been a substantial effort by both government and industry to develop underground salt deposits for various purposes. Salt deposits are being mined to provide long-term petroleum storage areas and disposal sites for radioactive waste. In addition, there has been extensive research into the use of salt caverns for storing compressed air as a means of energy storage. Electric power is used to compress and store air in these large caverns during periods of low energy demand. This compressed air is later used during periods of peak energy demand to drive turbines and electrical generators.
To form a large salt cavern, the salt is removed from an underground geological formation by solution-mining. A typical cavern being solution-minded is illustrated in FIG. 1. The cavern 5 is formed in a salt dome 7 which is covered by overburden and cap rock 9. The entire process is conducted through a small diameter, cased bore hole 11. Fresh water is injected through one of two concentric pipes 13, 15, called leaching strings, and the brine is extracted through the other pipe. The salt is dissolved near the point of injection of the fresh water. There is a third concentric pipe 17 called a blanket string which is used to inject a blanket material 19 that is less dense than water. The blanket stops the solutioning process above a certain height by pushing the level of water downward. Typically, either diesel oil or propane is used for the blanket material.
To mine a large cavern, the solutioning process may take several years. About ten times the volume of brine must be processed as the volume of the cavern. The extent of salt removal is controlled by monitoring the concentration of the brine that is removed. Periodically, the two leaching strings 13, 15 and the blanket string 17 are reset or the blanket pressure is changed in order to alter the shape of the area being excavated. At occasional intervals, the shape of the cavern is measured by retracting the three strings and lowering a measuring device into the cavern. In this manner, the diameter, depth, and wall uniformity are measured. When the desired shape of the cavern is obtained, the solution-mining process is stopped and water is pumped out.
A typical solution-mined salt cavern has a volume of about 6 million petroleum barrels, a diameter of about two hundred feet, and a height of approximately two thousand feet. The diameter of the bore hole is between ten and sixteen inches and is about one thousand feet in length. The bottom wall of the cavern is located at approximately three thousand feet below the surface.
At the present time the most common method for measuring the dimensions of a salt cavern is by sonar ranging with water propagated acoustic waves. This method is expensive and time consuming because it requires that the cavern be filled and emptied with water or brine. Typically, it takes several months to apply this method. In addition, an undesirable consequence of this technique is the further alteration of the cavern dimensions by additional solutionmining. The large liquid pressure load on the cavern walls also causes bulging. The diameter when measured with this technique is only an approximation of the diameter that exists when the cavern is filled with air. The hydrostatic pressure difference between the top and the bottom of a typical cavern filled with saturated brine is approximately twenty-eight atmospheres.
Another method of determining the dimensions of salt caverns is radar ranging in a pulse-echo mode using microwaves. Heretofore, these radar devices have been expensive and require an extensive amount of equipment that must be located on the surface. These systems are also not easily miniaturized for insertion through the long narrow bore hole.
Other prior methods have included the use of laser beams and air propagated acoustic waves. If a sufficiently powerful laser is used, the wall surfaces of the cavern reflect a measurable amount of light. However, in order to measure a variation in wall dimension of plus or minus one inch, the timing accuracy required for a laser beam is approximately 0.33.times.10.sup.-9 seconds. Although an elapsed time of this magnitude can be measured in the laboratory, such timing is difficult to perform under the rugged conditions of a drilling site. The air-salt interface also has a low coefficient of reflection to wavelengths between 0.2 and 20.mu.m. The use of air propagated acoustic waves has not been developed because of the problem of developing an acoustic transmitter that is sufficiently powerful to propagate waves across the cavern and yet small enough to fit into the bore hole. The use of air propagated acoustic waves is complicated by the increased attenuation of the waves as the wavelength is reduced to provide a measuring tolerance of one inch.
There is also a geometrical problem that occurs whether acoustic or electromagnetic waves are used. The walls of a salt cavern typically are mirror smooth. Because the walls are so smooth, both incident acoustic and electromagnetic waves are specularly reflected from the walls. When echo-ranging is performed, there is a high likelihood that the entire reflected beam will miss the receiver because the plane of the cavern wall is not orthogonal to the direction of propagation of the waves.